1. Technical Field of the Invention
The present invention relates to a method for recovering styrene monomers from a polystyrene resin by using a catalyst, and its objective is to provide a method for recovering styrene monomers from polystyrene resin wastes at comparatively low temperatures with high yield.
2. Prior Art
Polystyrene resins are one type of industrial plastics which were developed earliest among thermoplastic resins, and since they are superior in moldability and inexpensive, they have been used in a wide range of applications.
Among these, foamed polystyrenes, which have been widely used as packaging materials for electric appliances, AV apparatuses, etc., cushioning materials for fix boxes, glass products, etc. and trays and heat insulating and cold reserving containers for food, etc., are bulky and are not decomposed even if they are buried in the soil; therefore, it is difficult to prepare a site for disposal. Moreover, since they have high burning temperatures, they tend to damage incinerators, and since they produce dioxin when mixed with a chlorine-containing material, it is difficult to dispose them through incineration; thus, they have raised major problems in industrial waste disposal.
Under such circumstances, a number of arts for recovering aromatic hydrocarbon oils such as styrene monomers from polystyrene resins have been proposed, and examples of them include those arts disclosed in JP No. 283745/1996 (Tokukaihei 8-283745), JP No. 221681/1997 (Tokukaihei 9-221681) and JP No. 302356/1997 (Tokukaihei 9-302356).
Any of these arts thermally decompose polystyrene resins under the presence of a catalyst; however, since the thermal decomposing process is carried out by heating polystyrene resins to high temperatures of 350 to 430xc2x0 C., special plant facilities resistant to high temperatures are required, and such investments for facilities have increased initial costs.
Moreover, since recovered oils contain many components other than styrene monomers, the yield of styrene monomers becomes very low; for example, in the case of the method of JP No. 283745/1996, the ratio of styrene monomers contained in the recovered oils is as low as approximately 68 to 69%.
Here, JP No. 2545768 has disclosed a method for recovering styrene monomers from polystyrene resins by applying a heating process in a comparatively low temperature range of 300 to 350xc2x0 C.; however, even by the application of this method, the content of the styrene monomers contained in the recovered oils is in the range of 60 to 79%, which fails to achieve a satisfactory level.
The present invention has been devised to solve the above-mentioned problems, and its objective is to provide a method for recovering styrene monomers from polystyrene resins by using a catalyst, which can thermally decompose polystyrene resins at a comparatively low temperature so that it is possible to reduce the facility costs, etc., and which can recover oils that have less ratio of low molecular weight components and a high content of styrene monomers from the thermally decomposed vapor.
The present invention has been devised to solve the above-mentioned problems, and the invention according to claim 1 relates to a method for recovering styrene monomers from a polystyrene resin by using a catalyst, which obtains the styrene monomers by thermally decomposing the polystyrene resin, and which is characterized in that a sulfate is used as the catalyst.
The invention according to claim 2, which relates to the method of claim 1 for recovering styrene monomers from a polystyrene resin by using a catalyst, wherein the sulfate is a metal sulfate.
The invention according to claim 3, which relates to the method of claim 1 for recovering styrene monomers from a polystyrene resin by using a catalyst, wherein the sulfate is at least one member selected from the group consisting of magnesium sulfate, sodium sulfate, iron sulfate, manganese sulfate, zinc sulfate, aluminum sulfate, calcium sulfate, potassium sulfate and antimony sulfate.
The invention according to claim 4, which uses the method of claim 1 for recovering styrene monomers from a polystyrene resin by using a catalyst, wherein the heating temperature of the polystyrene resin is set to not more than 350xc2x0 C.